Process for the removal of vinyl chloride from polyvinyl chloride latexes and slurries with hydrocarbon compounds

ABSTRACT

Vinyl chloride is removed from polyvinyl chloride dispersions, latexes, and slurries by sparging them with a gaseous organic compound, such as methane, while they are maintained at subatmospheric pressure.

This is a continuation-in-part of our copending application, Ser. No.604,281, which was filed on Aug. 13, 1975.

This invention relates to a process for the purification of polyvinylchloride. More particularly, it relates to a process for the removal ofresidual vinyl chloride from polyvinyl chloride dispersions, latexes,and slurries.

When vinyl chloride is polymerized in an aqueous medium by suspension oremulsion polymerization techniques, there is obtained a dispersion,latex, or slurry that contains from 5% to 50% by weight, and in mostcases from 15% to 40% by weight of polyvinyl chloride. It also containsup to 5% by weight of residual vinyl chloride. Most of the unreactedmonomer is usually removed by heating the dispersion, latex, or slurryunder reduced pressure to about 65° C. As it is ordinarily practiced,this stripping procedure reduces the monomer content to about 1000 ppmto 20,000 ppm. Further processing yields dried products that may contain500 ppm or more of monomer.

In view of the recently-developed safety standards that require that theamount of vinyl chloride in polyvinyl chloride and in the atmospherethat workers breathe be maintained at very low levels, it is necessarythat the monomer content of the dispersion, latex, or slurry besufficiently reduced so that these requirements can be met.

A number of procedures for the removal of residual monomer frompolyvinyl chloride dispersions, latexes, and slurries have beenproposed, but none has proven to be entirely satisfactory. Proceduresthat involve heating them to temperatures above 65° C. or subjectingthem to conditions of high shear are effective in reducing the vinylchloride content to low levels, but they have an adverse effect on thestability and filterability of the dispersion or latex and on thefunctional performance of the dried product. In addition, the foamingthat often occurs at elevated temperatures creates serious processingproblems. Other procedures, for example, sparging with an inert gas orlow temperature stripping, either do not reduce the monomer content tothe desired very low level or are too slow to be practiced commercially.

In accordance with this invention, an improved process has beendeveloped for the removal of residual monomer from polyvinyl chloridedispersions, latexes, and slurries. This process rapidly reduces thevinyl chloride content of the dispersion, latex, or slurry withoutaffecting its stability and other properties or the properties of thedried polymer. Unlike many of the previously-known monomer removalprocedures, the process of this invention does not destabilize thedispersion, latex, or slurry thereby changing the particle sizedistribution of the polyvinyl chloride and so does not have adetrimental effect on the filterability of the dispersion, latex, orslurry or on the handling and drying characteristics of the wet polymercake.

In the process of this invention, a dispersion, latex, or slurry thatcontains from 5% to 50% by weight of polyvinyl chloride and from 1000parts to 20,000 parts by weight of vinyl chloride per million parts byweight of the dispersion, latex, or slurry is maintained undersubatmospheric pressure while it is sparged with a gaseous organiccompound. Following this treatment, the dispersion, latex, or slurry isfurther processed to yield a solid product that contains less than 10ppm and in most cases less than 1 ppm of vinyl chloride. The monomer andgaseous organic compound are removed through the vacuum system. Theymay, if desired, be recovered and recycled.

During the removal of vinyl chloride by the process of this invention,the dispersion, latex, or slurry is maintained at a temperature betweenabout 20° C. and the temperature at which its properties are adverselyaffected or at which the water in it will boil and at a pressure betweenabout 100 mm. and 600 mm. mercury absolute, preferably at a pressurebetween 125 mm. and 500 mm. mercury absolute.

The sparging may be carried out by any procedure that will providemaximum contact between the polyvinyl chloride particles and thesparging material. It is preferably done by passing the spargingmaterial through one or more diffusers that are located beneath thesurface of the dispersion, latex, or slurry.

A wide variety of organic compounds that are gases under the processingconditions employed can be used to remove vinyl chloride from polyvinylchloride dispersions, latexes, and slurries. The preferred organiccompounds are those that have boiling points below 0° C., that do notreact with any of the components of the dispersion, latex, or slurry andthat can be recovered readily from the exit gas stream. Examples of suchgases are methane, ethane, propane, butane, ethylene, propylene,butene-1, allene, ketene, methyl fluoride, ethyl fluoride methylenefluoride, fluoroform, carbon tetrafluoride, carbonyl fluoride,1,1,1-trifluoroethane, tetrafluoroethane, bromofluoromethane,chlorofluoromethane, chlorodifluoromethane, chlorotrifluoromethane,aminomethane, nitrotrichloromethane, carbonyl sulfide, dimethyl ether,and the like. A single gaseous organic compound or a mixture of gaseousorganic compounds, such as natural gas, can be used in the process ofthis invention. Especially good results have been obtained when thesparging gas was an aliphatic hydrocarbon having 1 to 4 carbon atoms ora mixture of such hydrocarbons.

The amount of the sparging material that is used and the rate at whichit is introduced are those that will effect rapid removal of vinylchloride without causing foaming or handling problems. They aredependent upon such factors as the amount of dispersion, latex, orslurry that is being purified, its vinyl chloride content, and theprocessing conditions that are employed. The optimum amount of organicgas and the optimum sparging rate for each dispersion, latex, or slurryand for each set of processing conditions can be readily determined bycarrying out a few preliminary experiments.

In a preferred embodiment of the invention, the process of thisinvention is used to remove vinyl chloride from dispersions and latexesthat contain polyvinyl chloride particles that range in size from about0.01 micron to 5 microns, and preferably from 0.05 micron to 2 microns.These dispersions and latexes cannot be subjected to procedures thatinvolve heating them to temperatures above 65° C. because such treatmenthas a deleterious effect on their stability, physical properties, andfilterability and on the quality of the dried polymer. In addition,foaming that occurs at elevated temperatures creates serious processingproblems. These dispersions and latexes are preferably maintained at atemperature between 40° C. and 60° C. and at a pressure between 100 mm.and 500 mm. mercury absolute while they are sparged. Among the sparginggases that are of particular value in the removal of vinyl chloride fromthese polyvinyl chloride dispersions and latexes are methane, propane,and natural gas. Particularly advantageous results have been obtainedwhen the dispersion or latex was maintained at a temperature between 45°C. and 55° C. and at a pressure between 125 mm. and 400 mm. mercuryabsolute while it was being sparged.

In another preferred embodiment of the invention, the process is used toremove vinyl chloride from aqueous slurries that contain polyvinylchloride particles ranging in size from about 5 microns to 200 microns.The slurries are preferably maintained at a temperature between about60° C. and their boiling points and at a pressure between 100 mm. to 600mm. mercury absolute while they are sparged. It is particularlypreferred that the slurries be maintained at a temperature between 70°C. and their boiling points at a pressure between 400 mm. to 500 mm.mercury absolute while they are sparged with an organic gas such asmethane, ethane, butane, or mixtures thereof.

As used herein, the term "polyvinyl chloride" includes both the highmolecular weight homopolymers of vinyl chloride and the high molecularweight copolymers formed by the copolymerization of vinyl chloride withan essentially water-insoluble ethylenically-unsaturated monomer that iscopolymerizable therewith. Suitable comonomers include vinyl acetate,vinyl propionate, vinyl butyrate, vinyl stearate, vinyl benzoate,ethylene, propylene, ethylacrylate, acrylic acid, acrylamide,acrylonitrile, methacrylonitrile, vinylidene chloride, dialkyl fumaratesand maleates, vinyl ethers, and the like. When one or more of thesecomonomers are used, the monomer component contains at least 70% andpreferably 80% to 90% of vinyl chloride.

The polyvinyl chloride dispersions, latexes, and slurries that aretreated in accordance with the process of this invention to removeresidual vinyl chloride from them may be prepared by the well-knownsuspension or emulsion polymerization processes. In the suspensionpolymerization process, the monomer is suspended in water by asuspending agent and agitation. The polymerization is initiated with asuitable free radical generating polymerization initiator, such aslauroyl peroxide, benzoyl peroxide, diisopropyl peroxydicarbonate,di-2-ethylhexyl peroxydicarbonate, tertiary butyl peroxypivalate,azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, and mixturesthereof. Suspending agents that may be used include methylcellulose,hydroxymethylcellulose, hydroxyethylcellulose, hydrolyzed polyvinylacetate, gelatin, methyl vinyl ether-maleic anhydride copolymers, andthe like. In emulsion polymerization processes, the polymerizationinitiator may be hydrogen peroxide, an organic peroxide, a persulfate,or a redox system. Surface-active agents, such as alkyl sulfates, alkanesulfonates, alkyl aryl sulfonates, and fatty acid soaps are used asemulsifiers in these processes. A preferred method of preparing thesepolymers is described in U.S. Pat. No. 2,981,722, which was granted toEnk et al. on Apr. 25, 1961. The reaction mixtures produced by theseprocesses are usually heated under vacuum to about 60° C. to 65° C. toremove most of the unreacted monomer from them. The resulting strippeddispersions, latexes, and slurries generally contain 5% to 50% andpreferably 15% to 40% by weight of polyvinyl chloride as particles thatrange in size from about 0.01 micron to 200 microns and from about 1000ppm to 20,000 ppm of vinyl chloride. Following the removal of vinylchloride from them by the process of this invention, the dispersion,latex, or slurry may be dewatered, for example, on a rotary drum filter,and then dried, or it may be spray dried. The polyvinyl chlorideprepared in this way has a vinyl chloride content of less than 10 ppm,and in many cases less than 1 ppm. It can be further processed withoutcreating a health hazard.

The invention is further illustrated by the following examples. In theseexamples and throughout this application, the terms "parts per million"and "ppm" mean parts by weight of vinyl chloride per million parts byweight of polymer.

EXAMPLES 1-3

A. A series of polyvinyl chloride latexes and dispersions was preparedby the following procedure:

An aqueous dispersion containing vinyl chloride, a mixture of lauroylperoxide and di-2-ethylhexyl peroxydicarbonate as the initiator,ammonium oleate as the emulsifier, and methylcellulose as the suspendingagent was homogenized and then polymerized to form a latex or dispersioncontaining between 30% and 33% of polyvinyl chloride in the form ofparticles ranging in size from 0.05 micron to 5 microns. The latex ordispersion was heated under vacuum at a temperature below 65° C. toreduce its vinyl chloride content to less than 10,000 parts per million.

B. One thousand gram portions of the latex or dispersion in 2-literflasks were maintained at subatmospheric pressure and sparged with a gasat the rate of 0.22 cubic feet per hour. Samples were taken periodicallyand analyzed to determine their monomer content. The conditionsemployed, the sparging gases used, and the results obtained are given inTable I.

The vinyl chloride and sparging gas were recovered from the exit gas andrecycled. The treated latexes or dispersions were filtered anddewatered. Each of the dried products contained less than 10 ppm ofvinyl chloride.

C. For comparative purposes, 1000 gram portions of the latex ordispersion in two-liter flasks were either sparged with a gas orsubjected to subatmospheric pressure for periods ranging from 2 hours to5 hours. Samples were taken periodically and analyzed to determine theamount of monomer that was present. The conditions employed and theresults obtained are given in Table I.

From the data in Table I, it will be seen that the process of thisinvention is substantially more effective than either vacuum treatmentor sparging at atmospheric pressure in removing vinyl chloride from apolyvinyl chloride latex or dispersion.

                  TABLE I                                                         ______________________________________                                                                     % of Vinyl Chloride                                                  Pressure Removed in                                                   Temp.   mm Hg.   Indicated Time                                   Sparge Gas  °C.                                                                            Absolute  2 Hrs.                                                                             4 Hrs.                                                                              5 Hrs.                               ______________________________________                                        Ex.                                                                           No.                                                                           1     Methane   55      230    88.9  99.7  >99.9                              2     Trifluoro-                                                                              45      230    88.0  97.5  99.2                                     chloro-                                                                       methane                                                                 3     Liquified 45      140    94.1  99.0  99.8                                     Natural                                                                       Gas                                                                     Comp.                                                                         Ex.                                                                           A     Methane   55      760    48.6  73.1                                     B     Trifluoro-                                                                              45      760    39.6  --    58.4                                     chloro-                                                                       methane                                                                 C     Liquified 45      760    47.1  --    67.7                                     Natural                                                                       Gas                                                                     D     None      45      230    76.1  --    81.3                               ______________________________________                                    

What is claimed is:
 1. The process for the removal of vinyl chloridefrom an aqueous dispersion, latex, or slurry that contains from about1000 parts to 20,000 parts by weight of vinyl chloride per million partsby weight of said dispersion, latex, or slurry and from 5% to 50% byweight of a vinyl chloride polymer selected from the group consisting ofpolyvinyl chloride and copolymers of vinyl chloride and at least onemonomer copolymerizable therewith that comprises maintaining thedispersion, latex, or slurry at a pressure between about 100 mm. and 600mm. mercury absolute while sparging it with a gaseous organic compoundselected from the group consisting of aliphatic hydrocarbons having 1 to4 carbon atoms, halogenated aliphatic hydrocarbons having 1 or 2 carbonatoms and 1 to 4 halogen atoms, and mixtures thereof until thedispersion, latex, or slurry contains less than about 10 parts by weightof vinyl chloride per part by weight of said vinyl chloride polymer. 2.The process of claim 1 wherein the sparging at a pressure between about100 mm. and 600 mm. mercury absolute is continued until the dispersion,latex, or slurry contains less than 1 part by weight of vinyl chlorideper million parts by weight of said vinyl chloride polymer.
 3. Theprocess of claim 1 wherein vinyl chloride is removed from a dispersionor latex that contains 15% to 40% by weight of a vinyl chloride polymer,said polymer being present in the dispersion or latex as particlesranging in size from 0.01 micron to 5 microns.
 4. The process of claim 3wherein the dispersion or latex is maintained at a temperature in therange of 40° C. to 60° C. and at a pressure between 100 mm. and 500 mm.mercury absolute during the sparging.
 5. The process of claim 3 whereinthe dispersion or latex is maintained at a temperature in the range of45° C. to 55° C. and at a pressure between 125 mm. and 400 mm. mercuryabsolute during the sparging.
 6. The process of claim 1 wherein thedispersion, latex, or slurry is sparged with an aliphatic hydrocarbonhaving 1 to 4 carbon atoms.
 7. The process of claim 6 wherein thedispersion, latex, or slurry is sparged with methane.
 8. The process ofclaim 1 wherein the dispersion, latex, or slurry is sparged with naturalgas.
 9. The process of claim 1 wherein vinyl chloride is removed from aslurry that contains from 15% to 40% by weight of a vinyl chloridepolymer, said polymer being present in the slurry as particles rangingin size from about 5 microns to 200 microns, by sparging the slurry withsaid gaseous organic compound while the slurry is maintained at atemperature between 60° C. and its boiling point and at a pressurebetween 100 mm. and 600 mm. mercury absolute.
 10. The process of claim 9wherein the slurry is maintained at a temperature between about 70° C.and its boiling point and at a pressure between 400 mm. and 500 mm.mercury absolute during the sparging.